1. Field of the Invention
The present invention relates to chopped carbon fibers suitable for producing a carbon fiber reinforced resin with a thermoplastic resin as the matrix, and also to a production process thereof. Particularly, it relates to a bundle of chopped carbon fibers produced from carbon fibers having a large number of filaments and large total fineness (so-called large tow), and to a production process thereof. In more detail, it relates to a bundle of chopped carbon fibers having excellent handling convenience such as flowability and bundle integrity useful as a reinforcing material of short fiber reinforced resin moldings, and to a production process thereof.
2. Description of the Related Arts
Since carbon fiber reinforced resins are excellent in strength, stiffness and dimensional stability compared to non-reinforced resins, they are widely used in various areas such as the office machine industry and the automobile industry. The demand for carbon fibers has been growing year after year, and is shifting from premium applications for aircraft, sporting goods, etc. to general industrial applications concerned with architecture, civil engineering and energy. So, the performance requirements for carbon fibers have become severe, and cost reduction is a major issue as important as higher performance. To meet such requirements, in recent years, carbon fibers (bundle) having a large number of filaments and large total fineness are being supplied to afford cost reduction.
Various methods are used for producing carbon fiber reinforced resins, and among them, the most popularly adopted method is to melt-knead about 3 to 10 mm long chopped carbon fibers together with resin pellets or resin powder by an extruder for pelletization (called the compounding process), and then to injection-mold the pellets into a product. The chopped carbon fibers used in such a process are usually bundled by a sizing agent for constant and stable supply, and the chopped carbon fibers bundled by the sizing agent are automatically continuously metered and supplied to an extruder by a screw feeder, etc.
An especially important property in that case is flowability, and unless that property is satisfied, the carbon fibers are blocked in the feeder hopper in an extreme case, not allowing processing.
In areas where powders are handled, it is known that the flowability of a powder in a hopper has correlation with various property values such as the coefficient of friction, the angle of repose, bulk density and form factor. For example, it is known that at a lower coefficient of friction, at a smaller angle of repose and at a higher bulk density, the flowability is higher. However, in the case of chopped fibers, the form factor of the chopped fibers more greatly affects these property values than in the case of a powder. So, for example, the angle of repose becomes varied, depending on measuring conditions, since an ideal conical form cannot be formed, and is affected by the size of the cone and the piling conditions (drop height, dropping velocity, etc.), and since also the measured value is affected by the quantity of the sample. After all, though property values can be judged to some extent, the final evaluation is effected by confirmation tests using the actual equipment in industrial production.
For improving the flowability and bundle integrity of chopped carbon fibers, various techniques are proposed in Japanese Patent Laid-Open (Kokai) Nos. 5-261729 and 5-261730, etc. in reference to publicly known powder handling techniques and techniques for glass fibers very similar to chopped carbon fibers. Chopped carbon fibers are larger than the grain size of a powder and are formed like rods or flakes, and carbon fibers are provided as a fiber bundle having a large number of filaments and large total fineness, unlike glass fibers processed after doubling fiber bundles that have a small number of filaments. So, the chopped carbon fibers are generally lower in flowability than chopped glass fibers. To replace chopped glass fibers in view of performance itself and cost performance, carbon fibers are required to have equivalent processability in the existing equipment to that of glass fibers without lowering productivity.
Conventional chopped carbon fibers have been produced from about 1,000 to 30,000 continuous filaments. However, for cost reduction of carbon fibers in recent years, a carbon fiber bundle having a larger number of filaments and larger total fineness than before is produced, and it becomes necessary to produce chopped fibers from such carbon fibers.
To produce a carbon fiber bundle having a larger number of filaments and larger total fineness, an original fiber bundle for producing the carbon fiber bundle is generally handled in a flat form for smoothly removing the reaction heat of oxidation.
A carbon fiber bundle having a large number of filaments and large total fineness has more flatness than the conventional carbon fiber bundle, and in addition, if the form of carbon fiber bundle is flat, the sizing agent is likely to permeate deep inside the bundle. For these reasons, if a process similar to the conventional process adopted for a carbon fiber bundle consisting of 1,000 to 30,000 filaments is adopted for producing chopped carbon fibers, the flatness adopted in the production becomes greater.
On the other hand, if the form of the carbon fiber bundle is flat, the chopped carbon fibers have low flowability and bundle integrity, disadvantageously.
If the sectional form of the bundle is made more circular, the bulk density of the fiber bundle becomes higher, causing the sizing agent to be less likely to permeate the fiber bundle deep inside, hence the bundle integrity becomes irregular. Furthermore, the shear force acting in the compounding process is likely to be so large as to open the fibers, and fiber balls are likely to be formed lowering flowability. Thus, in the transfer from the hopper of the compounding process to an extruder, such drawbacks as blocking are likely to occur.
As a general conventional method for obtaining chopped carbon fibers, at first carbon fibers (bundle) are immersed in a sizing agent, and bundled in a drying step, and subsequently the carbon fibers are chopped by a cutter in a continuous or discontinous line. On the other hand, as a general method for chopping glass fibers, a sizing agent is applied to melt-spun glass fibers, and the glass fibers are cut in a wet state, then being dried. If this method for chopping glass fibers is adopted, chopped fibers with higher bundle integrity can be easily obtained with a smaller amount of deposited sizing agent, and this method is adopted for carbon fibers in Japanese Patent Laid-Open (Kokai) Nos. 5-261729 and 5-261730. However, the carbon fiber bundle to be chopped by these techniques consists of about 12,000 filaments, and these techniques are not intended to process a carbon fiber bundle having a larger number of filaments and larger total fineness. Also for said chopped glass fibers, the fiber bundle in the step of applying a sizing agent consists of about 4,000 filaments, and it is not intended to process a thicker fiber bundle.
The present invention relates to a bundle of chopped carbon fibers excellent mainly in flowability and bundle integrity, used for making a carbon fiber reinforced composite.
In more detail, the present invention is intended to solve such problems as the necessity of using a cost-effective carbon fiber bundle having a larger number of filaments and larger total fineness as a raw material, and the decline of flowability and bundle integrity of chopped carbon fibers caused by the high flatness involved in the use of the cost-effective carbon fiber bundle.
The inventors studied variously to solve the above problems, and as a result, completed the present invention.
The chopped carbon fiber bundles of the present invention comprise a set of chopped carbon fibers impregnated with a sizing agent, the short fiber bundle pieces constituting a set having an average weight per unit length of 1.7 to 4 mg/mm in the fiber length direction and a coefficient of variation of 30 to 60% in the distribution of weight per unit length in the fiber length direction.
A preferable process for producing the chopped carbon fibers of the present invention comprises the steps of applying a sizing agent as a water dispersed sizing agent to a continuous carbon fiber bundle consisting of 20,000 to 150,000 filaments, controlling the packing density in a range of 5,000 to 20,000 D/mm, cutting the carbon fiber bundle in a wet state of 10 to 35 wt % in solution content at the time of cutting, and drying with vibration at a solution content of 15 to 45 wt % before drying.